Method for fabrication of electrodes

ABSTRACT

Described herein is a method to fabricate porous thin-film electrodes for fuel cells and fuel cell stacks. Furthermore, the method can be used for all fuel cell electrolyte materials which utilize a continuous electrolyte layer. An electrode layer is deposited on a porous host structure by flowing gas (for example, Argon) from the bottomside of the host structure while simultaneously depositing a conductive material onto the topside of the host structure. By controlling the gas flow rate through the pores, along with the process conditions and deposition rate of the thin-film electrode material, a film of a predetermined thickness can be formed. Once the porous electrode is formed, a continuous electrolyte thin-film is deposited, followed by a second porous electrode to complete the fuel cell structure.

RELATED APPLICATIONS

[0001] This application is a Divisional of Ser. No. 09/906,913 filedJul. 16, 2001 entitled “Method for Fabrication of Electrodes” byinventor(s) Alan F. Jankowski, Jeffrey D. Morse and Randy Barksdale.

[0002] The United States Government has rights in this inventionpursuant to Contract No. W-7405-ENG-46 between the United StatesDepartment of Energy and the University of California for the operationof Lawrence Livermore National Laboratory.

BACKGROUND OF THE INVENTION

[0003] The simplest fuel cell comprises two electrodes separated by anelectrolyte. The electrodes are electrically connected through anexternal circuit, with a resistive load lying in between them. Solidpolymer electrochemical fuel cells generally employ a membrane electrodeassembly, or “MEA,” comprising a solid polymer electrolyte membrane, or“PEM,” also known as a proton exchange membrane, disposed between thetwo electrodes. The electrodes are formed from porous, electricallyconductive sheet material, typically carbon fiber paper or cloth, thatallows gas diffusion. The PEM readily permits the movement of protonsbetween the electrodes, but is relatively impermeable to gas. It is alsoa poor electronic conductor, and thereby prevents internal shorting ofthe cell.

[0004] A fuel gas is supplied to one electrode, the anode, where it isoxidized to produce protons and free electrons. The production of freeelectrons creates an electrical potential, or voltage, at the anode. Theprotons migrate through the PEM to the other electrode, the positivelycharged cathode. A reducing agent is supplied to the cathode, where itreacts with the protons that have passed through the PEM and the freeelectrons that have flowed through the external circuit to form areactant product. The MEA includes a catalyst, typically platinum-based,at each interface between the PEM and the respective electrodes toinduce the desired electrochemical reaction.

[0005] In one common embodiment of the fuel cell, hydrogen gas is thefuel and oxygen is the oxidizing agent. The hydrogen is oxidized at theanode to form H⁺ ions, or protons, and electrons, in accordance with thechemical equation:

H₂=2H⁺+2e ⁻

[0006] The H ions traverse the PEM to the cathode, where they arereduced by oxygen and the free electrons from the external circuit, toform water. The foregoing reaction is expressed by the chemicalequation:

½O₂+2H⁺+2e ⁻=H₂O

[0007] Solid Oxide Fuel cells (SOFCs) operate using a mechanism similarto PEMs. The main difference is that instead of the electrolyte materialcomprising a polymer material capable of exchanging protons, theelectrolyte material comprises a ceramic material capable of exchangingelectrons.

[0008] Electrode layers must be porous in order to allow the fuel andoxidant to flow to the electrode-electrolyte interfaces. Typical fuelcells that use porous electrode materials are bulk structures thatrequire significant manifolding and pressures to readily deliver thefuel to the electrode-electrolyte interface. These porous electrodes areformed by pressing and sintering metal powders to promote adhesion, thensandwiching two such electrodes around an electrolyte layer to form afuel cell or in series to form the fuel cell stack. A method tofabricate porous electrodes that can reduce or remove the need for hightemperatures or high pressures to assist the flow of the fuel andoxidant to the electrode-electrolyte interface may be an importantcontribution to fuel cell technology.

SUMMARY OF THE INVENTION

[0009] Aspects of the invention include a method comprising the steps of

[0010] Simultaneously: (1) coating a topside of a porous host structurewith a plurality of conductive material particles and (2) flowing gasthrough a bottom side of the porous host structure to form a conductiveporous electrode layer on the topside.

[0011] Other aspects of the invention include an electrode comprising aconductive material having a plurality of pores, the electrode having apore size distribution wherein at least 90% of the total pore volume isin pores of diameter from about 10% below the mode pore diameter toabout 10% above the mode pore diameter.

[0012] A fuel cell comprising at least one electrode comprising aconductive material having a plurality of pores, the electrode having apore size distribution wherein at least 90% of the total pore volume isin pores of diameter from about 10% below the size of the mode porediameter to about 10% above the size of the mode pore diameter.

[0013] A fuel cell stack comprising at least one fuel cell having atleast one electrode comprising a conductive material having a pluralityof pores, the electrode having a pore size distribution wherein at least90% of the total pore volume is in pores of diameter from about 10%below the size of the mode pore diameter to about 10% above the size ofthe mode pore diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated into and form apart of the disclosure, are as follows:

[0015]FIG. 1 is an illustration of an electrode-electrolyte-electrodeportion of a fuel formed on a host structure that has been mounted to atemplate;

[0016]FIG. 2 is an illustration of the simultaneous processes of vacuumdeposition and gas flow that form a metal electrode;

[0017]FIG. 3 is an illustration of how a layer of metal greater inthickness than the diameter of the pore size of a porous host structurecan be formed;

[0018]FIG. 4 is an illustration of how the pore is pinched off whenforming an electrode after a desired thickness of deposited metal hasbeen achieved;

[0019]FIG. 5 is an illustration of a metal electrode formed by mountinga porous template to the porous host structure.

DETAILED DESCRIPTION

[0020] Fuel cells include, but are not limited to, an anode layer, anelectrolyte layer, a cathode layer and optionally catalysts to enhancereaction kinetics. Fuel cell stacks comprise two or more fuel cellsconnected either in series or in parallel. It is desirable to haveporous electrodes so that the fuel and oxidant can easily flow to therespective electrode-electrolyte interface without the need for hightemperatures or high pressures to assist the flow. Described herein is amethod to form a porous thin-film electrode structure. This approach canbe used for all fuel cell electrolyte materials that utilize acontinuous electrolyte layer. Moreover, the method can be used tofabricate porous electrodes useful in fuel cells such as, a solid oxidefuel cell (SOFC) and a proton exchange membrane fuel cell (PEMFC)sometimes referred to as a solid polymer fuel cell (SPFC). Powerdensities measured in output per unit area may be achieved up to about 1W/cm² for PEMFCs and up to about 2 W/cm² for SOFCs. Typically powerdensities range from about 0.1 W/cm² to about 0.4 W/cm². The poweroutput of these fuel cells ranges from about 0.1 Watts to about 50Watts. Typical outputs range from about 1 Watt to about 10 Watts.

[0021] A porous thin-film anode or cathode structure may be formed froma host structure or substrate having a high percentage of continuousopen porosity, e.g., greater than 40% by volume (measured by mercuryporosimetry). Examples of such substrates include anodized alumina,silicon that has been anisotropically etched, or a polycarbonate filmthat has been irradiated by heavy ions and selectively etched bypotassium hydroxide. The pore sizes in such a structure nominally rangefrom about 0.05 μm to about 1 μm in average cross-sectional diameter(measured by scanning electron microscopy or optical microscopy), areclosely spaced, and are continuous throughout the substrate/hoststructure.

[0022] Vacuum deposition techniques may be utilized to coat the surfaceof the host structure with conductive metals, polymers and/or ceramicmaterials to form the electrode. Some examples of electrode materialsinclude silver, nickel, platinum, and lanthanum-strontium-maganate.Under appropriate process conditions, such a conductive material can bedeposited to a thickness where the pores are not completely closed atthe top surface of the conductive coating. Subsequent deposition and/orapplication of a continuous electrolyte layer, and a complimentaryporous electrode can complete the fuel cell structure. If the newlydeposited electrode on the porous host structure closes off the porescompletely, then diffusion of fuel and oxidant through the electrode andelectrolyte layers will no longer be possible. The use of conventionalvapor deposition techniques necessarily limits the thickness of thedeposited electrode to a width on the order of the size of the diameterof the pores of the host structure. The electrode can have anexcessively high overall electrical resistance as a consequence of suchsmall dimensions. The resistance of the electrode can be reduced tosuitable levels for efficient conduction of current when a conductivefilm much thicker than the diameter of the pores of the host structurecan be deposited onto the host structure.

[0023] The thickness of the porous and conductive electrode can beincreased when a gaseous material, such as an inert gas, e.g., Argon, atabout 0.1 sccm (standard square centimeters per minute) to about 300sccm, is flowed through the pores of the host structure from thebottomside during the vacuum deposition process. The deposition rate andadditional process conditions for the conductive material enableapplication of an electrode layer that can be constructed to be muchthicker in size than the average cross-sectional diameter of the poresof the host structure, thereby significantly reducing the resistance ofthe electrode. For example, the poor performance that can result fromhigh current, breakdown voltage, resistive losses in thin (less thanabout 0.1 μm) metals are eliminated for conductive layers greater thanabout 0.15 μm in thickness.

[0024] In addition, electrodes having uniform pore size distributionsfrom about 0.1 μm to about 10 μm wherein at least 90% of the total porevolume is in pores of diameter from about 10% below the mode porediameter to about 10% above the mode pore diameter can be obtained. Modepore diameter is defined as the pore diameter occurring most frequentlyin any given porous electrode. Tapered pores can also be obtained withthis method wherein the size of the two pore openings can be tailored tospecific sizes. A tapered pore is a pore with the size of one openingsmaller than the size of the other opening. The sizes of the twoopenings can vary by up to a factor of 10. Features such as pore sizeand operating temperature will determine the rate at which fuel andoxidant can be passed through the fuel cell. For example, smaller poresizes can be desirable in a low temperature PEM cell that generates upto about 0.1 watts/cm², whereas larger pore sizes can be desirable in ahigh temperature SOFC that generates up to about 2 watts/cm². Thus, theability to tailor the pore size of electrodes to operating temperaturesand other parameters of fuel cells can create very efficient energysystems.

[0025] Referring to FIG. 1, a continuous electrolyte layer 2 ispositioned between two porous electrodes, such as anode 4A and cathode4B, forming an electrode-electrolyte-electrode portion of a fuel cell orfuel cell stack. Anode and cathode positions can be interchangeable. Oneembodiment of the method includes a porous host structure 6 having ahigh density of pores 7 already formed in it. In a further embodiment,host structure 6 may be mounted to a porous template 8. Pore sizes inhost structure 6 are typically between about 0.05 μm and about 1 μm,calculated as average cross-sectional diameter, whereas pore sizes intemplate 8 are larger than those of host structure 6, e.g., havingaverage cross-sectional diameters on the order of about 0.1 μm to about3 μm. The direction of fuel flow through the fuel cell as shown byarrows 10, is through pores 7 toward the electrode-electrolyte-electrodeinterface 5.

[0026] Furthermore, electrolyte layer 2 is an insulating material thatis ion-conducting or proton conducting. Formation of electrolyte layer 2can be accomplished by using a physical or chemical vapor depositionmethod and/or a laminate method. Examples of effective methods includesol-gel, plasma spray, dip coating, tape casting, and evaporation.

[0027] Referring to FIG. 2, the manner in which the conductive layer isformed is illustrated. Arrows 12 show the direction of gas flow throughpores 7. As conductive particles 14 are deposited by vacuum depositiontechniques, the gas flow through pores 7 causes a plurality ofconductive particles 14 to disperse away from pores 7. The direction ofvacuum deposition is shown by arrow 16. FIG. 3 is an illustration of theformation of electrode 4A obtained from the deposition of scatteredconductive particles 14 on host structure 6 (the direction of gas flowis shown by arrows 12 and the direction of vacuum deposition is shown byarrows 16).

[0028] Another embodiment of the method for depositing the electrode isillustrated in FIG. 4. After an electrode 18 of a desired thickness 19has been deposited on host structure 6, the rate of gas flow 12 throughpores 7 is reduced in order to allow a plurality of sputter depositedconductive particles 14 to narrow down or partially pinch off pores 7 atan orifice distal to host structure 6, i.e., to reduce the orificedimensions of pores 7 in order to create tapered pores 20.

[0029] Another embodiment of the method of the invention is illustratedin FIG. 5. Porous host structure 6 is mounted on template 8 whereintemplate 8 has a grid of pores 22 larger in average cross-sectionaldiameter than that of the pore sizes of host structure 6. Gas flow 12 isprevented from flowing through the pores of host structure 6 because thepores are blocked by grid 22. If the ratio of the area of the blockedpores to unblocked pores is small, the vacuum deposition rate is greateron the surface above grid 22 resulting in an electrode 4 having athicker layer of conductive particles at such areas 24. The grid patternis translated to areas of thicker electrode material on a surface havinglow resistance for conducting current efficiently away from the areas ofporous electrode.

[0030] While particular operational sequences, materials, temperatures,parameters, and particular embodiments have been described and orillustrated, such are not intended to be limiting. Modifications andchanges may become apparent to those skilled in the art, and it isintended that the invention be limited only by the scope of the appendedclaims.

The invention claimed is:
 1. An electrode comprising a conductivematerial having a plurality of pores, said electrode having a pore sizedistribution wherein at least 90% of the total pore volume is in poresof diameter from about 10% below the size of the mode pore diameter toabout 10% above the size of the mode pore diameter.
 2. The electrode ofclaim 10, wherein said pore sizes are in the range of about 0.1 μm toabout 10 μm as measured by scanning electron microscopy
 3. The electrodeof claim 10, wherein said pores are tapered having a first pore openingand a second pore opening, wherein said first pore opening is up toabout a factor of 10 smaller in size than said second pore opening,wherein said pore openings are measured by scanning electron microscopy.4. A fuel cell comprising at least one electrode comprising a conductivematerial having a plurality of pores, said electrode having a pore sizedistribution wherein at least 90% of the total pore volume is in poresof diameter from about 10% below the size of the mode pore diameter toabout 10% above the size of the mode pore diameter.
 5. The fuel cell ofclaim 13, wherein said pores are tapered having a first pore opening anda second pore opening, wherein said first pore opening is up to about afactor of 10 smaller in size than said second pore opening, wherein saidpore openings are measured by scanning electron microscopy.
 6. The fuelcell of claim 13, wherein the pore sizes are in the range of about 0.1μm to about 10 μm as measured by scanning electron microscopy
 7. A fuelcell stack comprising at least one fuel cell having at least oneelectrode comprising a conductive material having a plurality of pores,said electrode having a pore size distribution wherein at least 90% ofthe total pore volume is in pores of diameter from about 10% below thesize of the mode pore diameter to about 10% above the size of the modepore diameter.
 8. The fuel cell stack of claim 16, wherein said poresare tapered having a first pore opening and a second pore opening,wherein said first pore opening is up to about a factor of 10 smaller insize than said second pore opening, wherein said pore openings aremeasured by scanning electron microscopy.
 9. The fuel cell stack ofclaim 16, wherein the pore sizes are in the range of about 0.1 μm toabout 10 μm as measured by scanning electron microscopy